The present invention relates to a reflow apparatus for soldering and mounting electronic components to printed circuit boards.
Conventionally, ambient gas for heating or cooling printed circuit boards is separated for every predetermined temperature in a furnace of a reflow apparatus. The separated gas, while heated or cooled, is circulated in the furnace, thereby heating the printed circuit boards and melting the solder to solder the electronic components.
A conventional example of the reflow apparatus will be depicted with reference to FIG. 9.
In FIG. 9, inside a reflow furnace (a), an ambient gas for heating or cooling each printed circuit board is separated for every predetermined temperature and circulated while being heated or cooled in a plurality of adjusting/circulating sections (b). Consequently, the printed circuit board is heated and the solder is melted to solder the electronic components. A transfer means (f) holding the printed circuit boards with the electronic components thereon runs through the adjusting/circulating sections (b). The ambient gas is circulated by a sirocco fan (h) and heated by a heater (g) in the heating-type adjusting/circulating sections (b). The heated ambient gas is circulated as shown by an arrow (d). In the cooling-type adjusting/circulation section (b), meanwhile, the ambience gas is circulated by a fan (k) and cooled by heat exchangers (m). The cooled ambient gas is circulated as shown by an arrow (p). A feed port (i) is formed in the heating adjusting/circulating section (b) so as to send the pressured ambient gas into the furnace (a). The amount of the pressured ambient gas, when sequentially moved and circulated and finally flowed outside through an inlet (n) or an outlet (o) is regulated at residence sections (c).
In the above-described constitution of the conventional reflow apparatus, the ambient gas pressured to not lower than the atmospheric pressure, is supplied from the feed port (i). The amount of the ambient gas that is moved is regulated by the action of the residence sections (c). The ambient gas is sequentially moved to the adjusting/circulating section (b) at the side of the inlet (n) or outlet (o) and finally sent out to the air through the inlet (n) or outlet (o). During this time, the moving ambient gas is circulated by the sirocco fans (h) in the heating adjusting/circulating sections (b) as shown by the arrow (d) and heated by the heaters (g). On the other hand, the moving ambient gas is cooled by the heat exchangers (m) in the cooling adjusting/circulating section (b) and circulated by the fan (k) as is indicated by the arrow (p). The ambient gas eventually flows out into the atmosphere through the inlet (n) or outlet (o) because of the originally-added pressure. The ambient gas is collected by compulsory ducts (j).
According to the conventional apparatus, components of a flux vaporized in the furnace as the solder is heated are circulated along with the ambient gas. Therefore, the amount of the flux components in the ambient gas is gradually increased. When the flux components adhere to parts of the furnace, particularly where the temperature is low, the reflow apparatus operates poorly. Moreover, the flux components may adhere to the printed circuit boards, which deteriorates the quality of the produced circuit boards.
In setting the temperature of each adjusting/circulating section, if the temperature difference is large, the ambient gas in the adjusting/circulating section of a higher temperature inevitably flows and leaks to the adjusting/circulating section of a lower temperature, whereby the temperature at the lower side cannot be controlled and is raised excessively. This results in defective circuit boards. Although a fresh ambient gas of a low temperature, nitrogen gas in most cases, is arranged to be supplied to the adjusting/circulating section of the lower temperature in order to solve the problem of the above excessive rise of the temperature in a recently-employed arrangement, this requires a larger amount of the ambient gas and disadvantageously increases the running costs of the apparatus.